Cation-Pi cyclizations initiated by the disrotatory electrocyclic opening of cyclopropyl derivatives will be studied as a general method for the synthesis of alicyclic and heterocyclic compounds. This investigation will focus in particular on the utility of this strategy in the synthesis of polycyclic systems incorporating eight-membered carbocyclic rings. The application of this synthetic approach as a route to nitrogen heterocycles will be demonstrated in short and practical total syntheses of the pharmacologically interesting neurotoxic alkaloids anatoxin a and perhydrohistrionicotoxin. A new [4+4] annulation approach to eight-membered carbocycles involving the thermal combination of 1,3-dienes and cyclobutenones to afford 2,6-cyclooctadienone derivatives will be investigated. The application of this methodology to the synthesis of bicyclo[6.3.0]- and bicyclo[5.3.1]undecane derivatives will be examined. These systems are incorporated in the structures of several natural products including sesterterpenes of the ophiobolin family, the antibiotic pleuromutilin, and the antitumor diterpene taxol.